Short range RF monitoring system

ABSTRACT

A wireless short range radio-frequency master device adapted to create and maintain a portable private network of wireless short range radio-frequency slave devices wherein the master device is configured to detect and register suitable slave devices for a network, and is capable of determining the proximity of any registered slave device with respect to the master device in use, the master device further being adapted to enable a user to define two or more groups of registered slave devices selected from the total number of registered slave devices and to enable a user to select a defined group of such registered slave devices as an active group, thereby forming an active portable private network of wireless short range radio frequency devices comprising the master device and selected registered slave devices within the selected group.

TECHNICAL FIELD

The invention relates to a method and apparatus for monitoring a networkof wireless short range radio-frequency devices. In particular, but notexclusively, the invention relates to apparatus for forming a network ofitems which can be organised in groups and enable a user to determinethe presence and/or absence one or more of the items within the network.Additionally, the invention relates to apparatus for enablingdetermination of the proximity and/or orientation of a device within thenetwork relative to a master device.

BACKGROUND TO THE INVENTION

It is known for two or more wireless short range radio-frequencydevices, or Bluetooth (trademark) devices, to form a private networkknown as a piconet. A piconet comprises, a master device and up to amaximum (according to the Bluetooth standard) of seven active slavedevices.

FIG. 1 is an example of the prior art. There is shown a piconet A whichwill be used as a basis for the embodiment of the invention as describedbelow. The piconet A consists of a master device B, a maximum of sevenslave devices C, and RF connections for transfer of information Dbetween the master device B and the slave devices C. In the preferredembodiment the master device B is a known mobile telecommunicationsdevice which has a radio-frequency transmitter and receiver thatcomplies to known Bluetooth specifications, but may be any suitabledevice which has wireless short range radio-frequency capabilities, forexample but not limited to a personal portable computer, a watch, aWibree® transmitter etc. The preferred embodiment of the slave devices Cis a bluetooth tag, but may be any suitable device which has Bluetoothcapabilities, for example but not limited to a personal portablecomputer, mobile phone, a dongle etc. The master device B and slavedevice C are ordinary Bluetooth devices with the standard two partarchitecture, comprising the controller E and the Bluetooth stack F. Thecontroller E consists of the hardware such as the Radio FrequencyController (RF), a link controller (LC) and a link manager (LMP). TheBluetooth stack F consists of the known standard communicationprotocols, such as L2CAP, RFCOMM, HCI etc. to communicate with thecontroller E. In a piconet A, the master device B can transmit data D toany slave device C, but a slave device cannot transmit data to anotherslave device. The slave devices C that form the piconet A are known asactive, slave devices C that are known to the master device B, but donot form part of the piconet A are known as inactive or parked.

The use of piconets to form ad-hoc networks to transfer data betweendevices is well known, however there is currently no example of using amaster device to maintain and monitor a portable piconet by measuringthe distance between the master and slave devices. The Bluetoothstandard does not specify a mechanism for calculating the separation ofdevices and as such it is impossible to perform a single calculation tocalculate the distance between devices in a piconet that will work onall Bluetooth enabled devices. Furthermore, there are no examples of aportable device that is able to determine the bearings of a slaveBluetooth device with respect to the master device. The currently knownmethods for determining the bearings require triangulation between twoor more fixed devices to determine the location of a portable slavedevice.

SUMMARY OF THE INVENTION

To mitigate at least some of the problems in the prior art there isprovided according to an aspect of the invention a wireless short rangeradio-frequency master device adapted to create and maintain a portableprivate network of wireless short range radio-frequency slave deviceswherein the master device is configured to detect and register suitableslave devices for a network, and is capable of determining the proximityof any registered slave device with respect to the master device in use,the master device further being adapted to enable a user to define twoor more groups of registered slave devices selected from the totalnumber of registered slave devices and to enable a user to select adefined group of such registered slave devices as an active group,thereby forming an active portable private network of wireless shortrange radio frequency devices comprising the master device and selectedregistered slave devices within the selected group.

In a further aspect of the invention there is also provided a system forthe creation of and maintaining of a portable private network ofwireless short range radio-frequency devices, comprising a master deviceas set out in any of the above claims and one or more slave devices,that are enabled to form a portable private network when activated bythe master device.

In yet another aspect of the invention there is provided a method ofcreating and maintaining a portable private network of wireless shortrange radio-frequency device, comprising a master device and one or moreslave devices, the method comprising the steps of; detection of theslave devices by the master device, registration of the slave device tothe master device and assigning the slave device to one or more groups,selection and activation of a group of slave devices, the group definingthe active slave devices that form the portable private network.

In a further aspect of the invention there is provided a method fordetermining the separation between at least two portable wireless shortrange radio-frequency devices, comprising a master device and one ormore slave devices the method comprising the steps of; detection of oneor more slave devices within communication range of the master device,measurement of the received and transmitted signal strength between themaster and slave devices, determination of the range of the slavedevices with respect to the master device based on the measured signalstrength, where the signal strength is determined by a combination ofone or more of the following; a measure of the strength of the mastertransmitted signal as received by a slave device, a ratio of thestrength of the signal received by the slave device to the strength ofthe signal transmitted by the master device, a ratio of the strength ofthe signal received by the master device to the strength of the signaltransmitted by the slave device, a determination of the threshold ofdetection of a slave device by variation of the strength of the mastertransmitter signal, a determination of the path loss rate as decibelloss of signal strength between the master and slave devices, adetermination of the bit error rate by measure of number of packets ofdata lost between the master device and a slave device, a calibration ofthe change in signal strength received by a slave device due to a changein the separation between the master and slave devices, by measurementof the strength of the signal received by the slave device from themaster device at one or more known separations from the master device, acalibration of the slave device transmitter and receiver by querying thedevice for manufacturer information, comparing the response to a list ofknown previously calibrated devices.

In another aspect of the invention there is provided a system fordetermining the distance between at least two portable wireless shortrange radio-frequency devices, comprising a master device and one ormore slave devices, the master device being configured to detect one ormore slave devices within communication range of the master device, themaster device being further configured to measure the received and/ortransmitted signal strength between the master and slave devices, andbeing enabled to calculate the range between itself and the slavedevices based in the measured signal strength.

According to another aspect of the invention there is provided awireless short range radio-frequency master device for determining thepositions of one or more wireless short range radio-frequency slavedevices relative to the master device, wherein the master device isconfigured to assess the strength of the radio signal between itself anda slave device at a plurality of orientations, thereby enabling adetermination of the relative position of the slave devices with respectto the master device based on the relative signal strengths at differentorientations.

According to a further aspect of the invention there is provided amethod for determining the bearing of one or more wireless short rangeradio-frequency slave devices, comprising the master device and one ormore slave devices, the method comprising the steps of; the masterdevice assessing the strength of the radio signal between itself and aslave device at an initial orientation, the master device being rotatedto one or more secondary orientations with respect to the initialorientation and assessment of the strength of the radio signal betweenitself and a slave device at each of the secondary orientations,determining the bearing of the slave devices based on a comparison ofthe radio signal strengths at the initial and secondary orientations.

Preferably wherein the signal strength is determined by a combination ofa combination of one or more of the following; a measure of the strengthof the master transmitted signal as received by a slave device, a ratioof the strength of the signal received by the slave device to thestrength of the signal transmitted by the master device, a ratio of thestrength of the signal received by the master device to the strength ofthe signal transmitted by the slave device, a determination of thethreshold of detection of a slave device by variation of the strength ofthe master transmitter signal, a determination of the path loss rate asdecibel loss of signal strength between the master and slave devices, adetermination of the bit error rate by measure of number of packets ofdata lost between the master device and a slave device, a calibration ofthe change in signal strength received by a slave device due to a changein the separation between the master and slave devices, by measurementof the strength of the signal received by the slave device from themaster device at one or more known separations from the master device, acalibration of the slave device transmitter and receiver by querying thedevice for manufacturer information, comparing the response to a list ofknown previously calibrated devices.

There is also provided according to another aspect of the invention asystem for determining the bearing of one or more wireless short rangeradio-frequency slave devices relative to a master wireless short rangeradio-frequency device, comprising a master device and one or more slavedevices, the master device being configured to assess the strength ofthe radio signal between itself and a slave device at a plurality oforientations, the master device being enabled to determine the relativeposition of the slave devices with respect to the master device based ona comparison of the signal strengths at different orientations.

Further aspects and/or features of the invention are further set out inthe other appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects, features and advantages of the invention will beapparent from the following description of preferred embodiments,presented by way of example only, and by reference to accompanyingdrawings wherein:

FIG. 1 is an example of a piconet, in the prior art;

FIG. 2 is an example of a master device, with several slave devices thatare assigned to groups and a user selected group of active slave devicesthat form a piconet;

FIG. 2 a is an example of a display of the preferred embodiment allowinga user to select the group of slave devices to form a piconet;

FIG. 3 is an example of the process of the transmission of a packet ofdata between a master and slave device to determine the separationbetween the devices;

FIG. 4 is a flow diagram representing the steps of the formation of auser defined group, the selection of a user defined group to form apiconet, and monitoring of the devices that form the piconet;

FIG. 5 is a flow diagram outlining the steps for calculating theseparation between the master and a slave device;

FIG. 6 is a representation of the process of determining the location ofa slave device by measurement of the signal strength at differentorientations of the master device.

FIG. 7 is a flow diagram representing the steps of determining thebearing of a slave device by measurement of the signal strength atdifferent orientations of the master device; and

FIG. 8 is an example of a display in the preferred embodiment of amaster device showing the bearing and separation of a slave device withrespect to the master device.

DESCRIPTION OF THE EMBODIMENT

FIG. 2 shows an example of the grouping of slave devices 14 andactivation of a group of slave devices 14 to form a piconet 10 in thepreferred embodiment. There is shown the master device 12, the slavedevices 14, which are known to the master device 12 through knownBluetooth standard detection techniques, the slave devices 14 areregistered in three groups, train 22, home 24 and office 26, the activedevices that form the piconet 10 and the transfer of data 16 between themaster device 12 and the devices in the piconet 10. The devices thatform the train group 22, wallet, keys and laptop are active and form thepiconet 10. All three items are also multiply defined, with all threeitems in the home group 24 and the wallet and the laptop in the officegroup 26. The remaining items in the home group 24 and office group 26are inactive and do not form part of the piconet 10. The master device12 in the preferred embodiment is a mobile telecommunications devicecomprising an antenna and controller adapted to communicate with localdevices using the Bluetooth standard.

Beneficially, such a master device 12 comprises a display 13 and otheruser interface elements such as a keypad to enable a user to interactwith the master device 12.

In the preferred embodiment the master device 12 is enabled to allow auser to select which slave devices 14 or group of devices 22, 24, 26form a piconet 10. In the example in FIG. 2 a user has activated thetrain group 22. The master device 12 therefore only transmits andreceives data 16 from the slave devices 14 in the train group 22. Theuser may for example, deactivate the train group 22 and activate theoffice group 26, in this case the piconet 10 would consist of slavedevices 14 called wallet, coat, laptop, hat and PDA. In the preferredembodiment up to a hundred different slave devices 14 may be registeredto the master device 12, though in other embodiments more slave devices14 may be registered, but only a maximum of seven may be active at anyone time.

FIG. 2 a shows an example of an interface of the preferred embodimentthat allows a user to register a slave device 14 and to activate a groupof slave devices 14 to form a piconet 10. There is shown an example of aregistration screen 32 and a group status screen 38. In the preferredembodiment both screens would be shown on the display 13 of the standardmobile telecommunications device and any user inputs would occur byknown means such as, but not limited to, keypad input, touch screenrecognition, voice recognition etc. The skilled person would understandthat the interface is not limited to be shown on the display 13 of astandard mobile telecommunications device but may be on other forms ofdisplay and that the screens shown are examples and that other featuresmay be displayed. The registration screen 32 is enabled to allow a userto name a slave device 14 in input field 34 and assign a sensitivity andalarm type 36 for the slave device 14. The sensitivity and alarm type 36of the slave device 14 allows the user to personalise the monitoring ofeach slave device 14. A device which is not expected to be moved may beassigned a high sensitivity. The alarm type 36 may indicate what type ofmonitoring occurs, for example an alarm classified as Absent istriggered when the slave device 14 goes out of range of the masterdevice 12, Threshold is triggered when the signal received by the slavedevice 14 drops below a given value, Motion is triggered when thedifference between the previous sample and the current one exceeds avalue. The group status screen 38 is enabled to allow a user select themonitoring status of the group 39, which would form the piconet 10.

Referring to FIG. 3 once a piconet 10 has been formed, the master device12 monitors the separation 46 of the active slave devices 14 that formthe piconet 10 with respect to the master device 12. FIG. 3 is arepresentation of the process of a master device 12 querying a slavedevice 14 in order to calculate the separation 46 between the devices.There is shown the master device 12, the slave device 14, a packet ofdata 42 sent from the master device 12 to the slave device 14, thetransfer of data between the master device to the slave device 44 andthe separation of the master device and slave device 46. The packet ofdata 42, comprises a payload header 48, the payload 50 and access code51, the payload header 48 comprises an address 52, the packet length 54and further information 55 as determined by the Bluetooth standard. Theslave device 14 comprises a controller 56 and a Bluetooth stack 58. TheBluetooth standards define the protocols for transmitting data betweendevices but do not define a standard for assessing proximity and thereis no single calculation to determine the separation 46 between any twodevices. The invention in the preferred embodiment therefore defines ameasured proximity (mProx) which is dependant on the hardware of thedevices which may be converted to an absolute value, the calculatedproximity (cProx). The calculated proximity is defined ascProx=normalise(damping (mProx)), and the calculation of thenormalisation factor, damping and measured proximity are describedbelow. Damping is used to correct for variations in the signal strengthdue to factors such as interference of the transfer of data 44,frequency hopping, reflections etc. The damping algorithm observes thehistorical sequence of values and assesses whether the current value isa genuine change or a spurious result. In a preferred embodiment thedamping algorithm is one that is known in the art for oscillatingsystems such as those found in amplifiers. The algorithm calculates amean and standard deviation values from the historical data and appliesthese values as a multiplier to the most recent measured value of signalstrength. In further embodiments other suitable known methods forcalculating the damping of the signal may be used.

Spurious results are damped out in the calculation, but retained in thehistorical data, since the damping decision may be subsequently revised.In the preferred embodiment, normalising takes the result of damping andattempts to match it to a ten point proximity scale. Preferably, theuser has performed a calibration of the master device 12 and each of theslave devices 14. To calibrate the master device 12 and a slave device14, the user separates the master device 12 and slave device 14 deviceby a predetermined distance and measures the signal strength received bythe slave device 14 at the known separation 46. The strength of thesignal received at the known separations and at the known transmissionstrengths, as used to calibrate the normalisation of the signal. As thefall-off of the signal strength is non-linear, a ten point scale tomodel the fall-off of signal strength with distance is calculated andused as the normalisation function. A mathematical curve is fitted tothe data points to allow the interpolation of other values. Thoughothers means for modelling the loss of signal strength with distance maybe used.

The measured proximity is calculated using the properties of the packetsof data 42 transferred between the master device 12 and the slave device14. Each packet of data 42, comprises a payload header 48, the payload50 and access code 51. The payload header 48 contains informationregarding the payload 50, including packet length 54 and the address 52of the slave device the packet is being sent to and further information55 as determined by the Bluetooth standard. From the informationcontained in the payload header 48, a comparison of the strength of thesignal received by the slave device 14 to the strength of the signaltransmitted can be made and therefore an estimate of the separation 46made. In the preferred embodiment seven base algorithms to determine theseparation are available. Each is based upon a different measurableparameter. These algorithms are used in different combinations with eachother to calculate a value for mProx. This compensates for hardwaredifferences and variations consequent upon the power saving strategiesused by different Bluetooth devices. The base algorithms used are shownbelow, but it is understood that a person skilled in the art may useother valid algorithms to provide a measure of the signal strength andtherefore device proximity.

-   -   a) Contact error—detection of whether the slave device 14 is in        response range of the master device 12, thereby providing a        limit of the distance of separation 46 between the master device        12 and slave device 14.    -   b) Received signal strength—Calculation based upon the strength        of signal received by the slave device 14. The Bluetooth        standards define optimum signal strength, known as the ‘Golden        Range’. The chipset returns a value of if the signal is within        this range, otherwise it returns an integer indicating (in dB)        the distance above or below the range that the signal lies.    -   c) Path loss—The further information 55 in the payload header        48, may be configured to contain the transmission strength of        the signal. A comparison of the transmission strength of the        signal to the strength of the signal received at the Bluetooth        stack 58, gives a measure of the signal strength lost along the        path 44.    -   d) Master power ramping—Power on the master transmitter is        varied and used with any of the above calculations to calculate        the measured proximity at various transmitter powers to refine        the value of mProx.    -   e) Data contact error—By decreasing the power of the transmitter        of the master device 12, a determination of the threshold of the        signal strength required for the slave device 14 to cease        receiving packets of data 42 from the master to the slave can be        made.    -   f) Data frame error—It is known to calculate the frame error        rate in a packet of data 42. By calculating the blocks of data        in the payload 50 lost in a single packet of data 42 due to        framing errors, a frame error for each packet of data 42 may be        calculated. This method is further refined by varying the signal        strength sent by the master device 12, and calculating the frame        error for each transmitter signal strength.    -   g) Data bit error—It is known to calculate the error rate in a        packet of data 42 as received by a slave device 14. Information        stored in the payload header 48, may include the packet length        54 transmitted, an error rate may be determined by a comparison        of the packet length received by the slave device 14, with the        packet length transmitted, which would be stored in the payload        header 54. Other methods for determining the error rate such as        cyclic redundancy check, may be used to provide a measure of the        error rate.

By calculating the bit error rate at different settings of thetransmitter of the master device 12 a measure of the proximity can bemade.

FIG. 4 is a flow diagram of the process 100 of the registration of slavedevices 14, formation of a piconet 10 and monitoring of the slavedevices 14 that form the piconet 10. All Bluetooth devices in range ofthe master device 12 are detected, using known protocols as defined bythe Bluetooth standard at step S102. Each detected device is checked tosee if it is registered with the master device 12 at step S104, if anunregistered device is detected, the user is queried as to whether theuser wishes to register the slave device 14 at step S106. If the userwishes to register the slave device 14, the user is presented with theregistration screen 32, where the user is able to add the device to oneor more groups at step S108. The user selects which group they wish toactivate and monitor at step S110 using the group status screen 38thereby activating the slave devices 14 to create the piconet 10 at stepS112. The master device 12 monitors the slave devices 14 that form thepiconet 10 by measuring their separation 46 from the master device 12 atstep S114. Determination of the separation 46 of the master device 12and the slave device 14 occurs as described above. The separations areassessed at step S116, to ensure that all active slave devices 14 thatform the piconet 10 are within a predetermined user defined range. Ifone or more slave devices 14 are outside of the predetermined range, orundetectable by the master device 12 the user is notified at step S118.Notification, in the preferred embodiment is via an audible alarm,though other means such as a visual alarm on the display 13, textmessage to the user etc. may be used.

FIG. 5 is a flow diagram of the process 200 to determine the separation46 of a slave device 14 from the master device 12. A packet of data 42is transmitted from the master device 12 to the slave device 14 at stepS202. The calculations of the measure proximity and separation 46between the devices using the methods as described above are made atstep S204. The person skilled in the art would appreciate that any sucha calculation of the separation would also return a measure of the errorin the calculation. In a preferred embodiment the signal strength anderror measures detected received from the hardware will have a tolerancewhich can either be determined directly from the chipset in the masterdevice 12 or assigned as part of the calibration process of a givendevice as described above with reference to FIG. 3. This is preferablyexpressed as a percentage +/− variation in the actual value. Thesetolerances are preferably combined for use in the proximity calculation,and the errors from the calculation are preferably combined with thetolerances to determine a percentage error range for the resultingvalue. In further embodiments other known suitable methods forcalculating the size of the error based on the strength of the signalreceived and method of calculation are used. Those skilled in the artwill understand that the error determination is largely based on themethod and hardware used in the embodiment. The size of the error wouldbe queried at step S206 and if it is above a pre-determined tolerancethen further calculations of the separation 46 are made at step S208until such a time that the error is within an acceptable limit. Theseparation 46 may be refined using the same or a different method thanin step S204.

The calculated separation 46 may then be displayed at step S210 on thedisplay 13, for example on the interface 70 shown in FIG. 8.

FIG. 6 is a representation of the method 60 used to determine thebearing of a slave device 14 with respect to the master device 12. Thereis shown the master device 12, at an initial orientation 62, atsecondary orientations 64, 66, 68 and the slave device 14, whichcomprises the controller 56 and the Bluetooth stack 58. Data istransmitted from the master device 12 to the slave device 14. The masterdevice 12 is at an initial orientation 62 and a calculation of thesignal strength is made. The signal strength is calculated using one ormore of the methods described above, though other methods forcalculating the signal strength are acceptable. In the preferredembodiment the master device 12 is rotated through 90 degrees to asecondary orientation 64 and the signal strength is calculated at thissecondary orientation 64. Once the signal strength has been calculatedthe master device 12 is further rotated to secondary orientations 66 and68, and the signal strength calculated at each of these orientations.The bearing of the slave device 14 with respect to the initialorientation 62 of the master device 12 is given by Bearing=arctan((s.o.s64-s.o.s 68)/(s.o.s 62-s.o.s 66)) where s.o.s is the strength of thesignal at the orientations shown in FIG. 6. The person skilled in theart will appreciate that this method may be adapted to incorporate anynumber of orientations greater than one, and that the differencesbetween distinct orientations need not be 90 degrees.

FIG. 7 is a flow diagram of the process 300 used to determine thebearing of a slave device 14 with respect to the initial orientation 62of the master device 12. The calculation of the strength of the signaloccurs at step S302. The master device 12 is rotated to a secondaryorientation and the signal strength at the secondary orientation iscalculated at step S304. A comparison of the signal strengths at thedifferent orientations is made at step S306 and a bearing determined.The person skilled in the art would appreciate that any such calculationof the bearing would be subject to an error. The calculation of theerror is preferably calculated by the same method as for calculating theerror in the distance as described above with reference to FIG. 5,though other methods of error calculation may be used. The size of theerror is queried at step S308 and if the error is above a pre-determinedtolerance then the master device 12 is rotated to another distinctsecondary orientation and the signal strength is assessed at step S304.The process continues until such a time that the bearing calculated isof the desired accuracy. In the preferred embodiment the calculatedbearing is displayed on the display 13 of the master device 12, at stepS310.

FIG. 8 shows an example of the user interface screen that would be shownon the master device 12. There is shown the separation measure screen70, with the active tags 72 and an indicator showing their separationfrom the master device 74. There is also shown, a direction indicatorscreen 76, with an arrow 78 indicating the bearing of a slave device 14with respect to the initial orientation 62 of the master device 12. Inother embodiments the separation 46 may be represented to the user by anaudible indicator, such as an alarm which varies in volume dependent onthe separation between the master device 12 and the slave device 14, orthe size of the arrow 78 may also be used to indicate the separation 46between the master device 12 and the slave device 14.

Whilst the above embodiments have been described in the context of theirapplication for use in a mobile telecommunications device for which theinvention is particularly advantageous, embodiments of the invention maybe applied in any system that is Bluetooth enabled. Furthermore, aperson skilled in the art would be aware that the above embodiment wouldalso be applicable to a scatternet, where a slave device 14 maysimultaneously be a master device 12 for another piconet 10, therebyallowing the monitoring of more than seven active slave devices 14.

The invention claimed is:
 1. A wireless short range radio-frequency master device being a mobile telecommunications device adapted to maintain a portable private network of wireless short range radio-frequency slave devices, wherein the master device is configured to: receive, at a plurality of orientations of the master device, a plurality of signals from two or more slave devices, wherein the plurality of signals are transmitted using variable strengths, determine, by comparing strengths of the plurality of signals, a calculated proximity of the two or more slave devices with respect to the master device, wherein the calculated proximity is associated with a physical distance between the master device and a slave device, determine from differences in the strengths of the plurality of signals, a direction of the two or more slave devices relative to the master device, enable definition of a set of slave devices selected from the two or more slave devices, and based on the determined calculated proximity and the determined direction, enable a user to monitor members of the set.
 2. A wireless short range radio frequency master device as in claim 1, wherein the master device is adapted to inform the user if any of the slave devices within the set is not within a predetermined proximity to the master device.
 3. A wireless short range radio frequency master device as in claim 1, wherein the master device is configured to display the status of the members of the set.
 4. A wireless short range radio frequency master device as in claim 1 that conforms to the Bluetooth standards.
 5. A wireless short range radio frequency master device as in claim 1, wherein the determination of the calculated proximity is based on historical signal strength of the plurality of signals transmitted using variable strengths.
 6. A wireless short range radio frequency master device as in claim 1, wherein the direction of the at least two slave devices relative to the master device is other than a direction associated with any of the plurality of orientations.
 7. A method of maintaining a portable private network of wireless short range radio-frequency devices, comprising a master device and slave devices, the method comprising the steps of: receiving, at a plurality of orientations of the master device, a plurality of signals from two or more slave devices, wherein the plurality of signals are transmitted using variable strengths, determining, by comparing strengths of the plurality of signals, a calculated proximity of the two or more slave devices with respect to the master device, wherein the calculated proximity is associated with a physical distance between the master device and a slave device, determining from differences in the strengths of the plurality of signals, a direction of the two or more slave devices relative to the master device, enabling definition of a set of slave devices selected from the two or more slave devices, and based on the determined calculated proximity and the determined direction, enabling a user to monitor members of the set.
 8. The method of claim 7 further comprising determining a signal strength of the plurality of signals, wherein the signal strength is determined by a combination of one or more of the following: a measure of the strength of the master transmitted signal as received by a slave device, a ratio of the strength of the signal received by the slave device to the strength of the signal transmitted by the master device, a ratio of the strength of the signal received by the master device to the strength of the signal transmitted by the slave device, a determination of the threshold of detection of a slave device by variation of the strength of the master transmitter signal, a determination of the path loss rate as decibel loss of signal strength between the master and slave devices, a determination of the bit error rate by measure of number of packets of data lost between the master device and a slave device, a calibration of the change in signal strength received by a slave device due to a change in the distance between the master and slave devices, by measurement of the strength of the signal received by the slave device from the master device at one or more known separation distances from the master device, and a calibration of the slave device transmitter and receiver by querying the device for manufacturer information, comparing the response to a list of known previously calibrated devices.
 9. A method according to claim 7, wherein the direction of the at least two slave devices relative to the master device is other than a direction associated with any of the plurality of orientations.
 10. A method as in claim 7, wherein a number of the plurality orientations is four and the angle of rotation between each orientation is approximately 90 degrees.
 11. A method as in claim 7, further compromising determining an error range of the calculated proximity.
 12. A non-transitory computer-readable medium having encoded thereon computer readable instructions which instructions when implemented enable the determination of the calculated proximity of one or more slave devices by the master device according to the steps of the method of claim
 7. 13. A system for maintaining a portable private network of wireless short range radio-frequency devices, comprising a wireless short range radio-frequency master device and wireless short range radio-frequency slave devices wherein: the master device is capable of receiving at a plurality of its orientations a plurality of signals from two or more slave devices, wherein the plurality of signals are transmitted using variable strengths, the master device is capable of determining, by comparing strengths of the plurality of signals, a calculated proximity of the two or more slave devices with respect to the master device, wherein the calculated proximity is associated with a physical distance between the master device and a slave device, the master device is capable of determining from differences in the strengths of the plurality of signals, a direction of the two or more slave devices relative to the master device, the master device is capable of enabling definition of a set of slave devices selected from the two or more slave devices and based on the determined calculated proximity and the determined direction, the master device is capable of enabling a user to monitor members of the set.
 14. The system of claim 13, wherein the master device is configured to measure a signal strength of the plurality of signals, and to calculate the distance to the slave devices based on the signal strength.
 15. The system of claim 13, wherein the direction of the at least two slave devices relative to the master device is other than a direction associated with an of the plurality of orientation. 